There are a number of forum topics and Frequently Asked Questions around discussing the different software tools available to keep the devices/hosts at a lower system temperature than otherwise would occur while crunching. Before summarizing them here, let's emphasize that all modern computers have automatic temperature regulation, single or combined by:

Lowering the CPU speed

Increasing the fan speed

Thermal shutdown protect if it gets too hot.

That makes crunching up front already inherently save, but BOINC itself can be set to only use a % of the CPU time. The WCG default preset profile setting is 60% for the extra comfort feeling and chosen as the level that serves the broadest range of laptops that tend to get warmer, where desktops hardly react.

The experience can be enhanced though since the BOINC CPU% use control is rather crude. You can add one or more tools/utilities for the days that it gets really hot and you don't want to crank up the Air Conditioning. Not endorsed by WCG, they are trustworthy. To name:

• Windows
  • Threadmaster GUI , a tool that allows to set a % of CPU time on a "per science" basis. Very smooth, but static as it does not respond to increased or decreased temperature. TM allows to set higher percentages if it's known that a science runs cooler than others, yes they are there. It can control any service or application's CPU use and works across all Windows versions upward from W2K and used by professionals as load balancers on servers.
    
    
  • TThrottle , also a tool to regulate CPU % use, but it's general and active. Set a target temperature and the software will automatically control the amount of CPU time used by the Science Application so it will never get warmer than wanted. The CPU itself continues to run at full cycles, so the user won't notice anything. Able to act on temp readings for up to 8 cores and 3 GPU's. It was specially developed to co-operate with BOINC and only control the sciences' use of CPU time (though it can control other software too). So far only build for windows, all versions.
    
    NB: As of version 1.57 the tool is accompanied by a signed driver so Vista and Windows 7 wont have UAC issues during automated start up.
    
    
  • SpeedFan , another tool, but it regulates the fan speed based on target temperature and much more refined than the one coming build into the hardware. Does not work on most laptops! For Windows.
    
    
  • TPFanControl Is able to control fan speed on some brand laptops which SpeedFan was not enabled for.
    
    
  • Process Lasso , slightly different as here you can force certain hot running sciences to combine on specific cores, an automated affinity control. Project X could generate 5 or more C's increase, so by combining several of the same science onto one core, each is only getting it's relative share of 1 core's cycles. For Windows.
    
• MAC
  • SMC Fan Control for Intel MACs. It will show you the temp and adjust your fan speed accordingly.
    
    
  • iStat Pro, A extensively configurable widget that allows monitoring every aspect of the system. It features 9 sections that reports on everything from cpu, memory and disk usage to which processes are using the most cpu.
    
    
  • iStat Nano, An advanced system monitor in a tiny package. iStat nano is a system monitor widget with animated menus and transitions.
  
  
• Linux
  • lm-Sensors on Ubuntu can monitor CPU and other system temperatures, fan speeds, and other system data.
    
    
  • Ksensors , GUI front end to lm-Sensors which can be obtained via the Synaptic Package Manager
    
    
  • GKrellM Monitors all kind of temperature information and can even send warning mails
    
    
  • pwmconfig(8) Allows interactive writing of fancontrol scripts, tapping into lm-sensors
    
    
  • fancontrol(8) taps into lm-sensors and pwmconfig(8)

There are some other tools to report on CPU, GPU and general system temperatures, usually sitting in the start bar system tray.

SpeedFan (mentioned above)

CoreTemp , displaying in system tray for each CPU core what momentary temperature each is.

OCCT , can run and log a temperature profile for a set time. Can be hooked into the SpeedFan temp reading driver.

Do note that system come with tools as SpeedStep (for Intel) and Cool & Quiet (for AMD). You will want to see how these are set so they don't behave contra to expectation in combination with the above software.

NB: This FAQ is a Work in Progress, so stay tuned for more throttle and temperature regulation and monitor additions, for Linux and Mac too, if they are free!

AND Do keep all the ventilation free of dust and obstructions, to include the cooling elements on the CPU.